Slugs and snails are major pests of agriculture in many parts of the world. Their biology tends to favour activity in moist conditions such as habitats which are continually wet and temperate regions, especially during rainy summers and autumns. As a consequence, their potential for damage is considerable.
The-ecologies of different types of molluscs, which can be either terrestrial or aquatic, are very different and they usually require different types of treatment. The snail species Theba pisana, Cernuella virgata, Helix aspersa and Achatina spp and the slug species, Arion hortensis, Milax budapestensis, Deroceras reticulatum and Limax maximus are of particular interest as targets. The common garden snail, Helix aspersa, and the grey field slug, Deroceras reticulatutni, are common garden pests throughout temperate Australia. These pests have established themselves in many parts of the world, adapting to a wide range of climatic conditions. They rarely increase in numbers above 20 per square meter but cause darnage by feeding, with minor damage due to the mucus on which they move. Helix aspersa is, in general, a nocturnal feeder and in the daytime remains hidden on the underside of leaves, under rocks or in cracks in the soil. It flourishes in moist conditions. On the other hand, there are a group of snails which have been introduced into Australia in the twentieth century. The areas in which these are pests (often over 200 per square meter) are still expanding. These are the white Italian snail, Theba pisana and the vineyard or Mediterranean snail, Cernuella virgata, which can survive long hot summer temperatures by aestivating on weeds and fence posts, retreating into their shells and secreting a hard mucous film to reduce moisture loss and rest. These snails are of some concern to Australian farmers because they also aestivate on the heads of cereal stalks in November and December and during harvest, they clog up the machinery and contaminate the grain, making it either unacceptable or forcing it to be downgraded. There are very significant variations of the pest numbers and in a bad year it is uneconomic to harvest substantial areas of crops. In cold climates, Theba pisana hibernates in winter. The slug, Deroceras reticlilatum, is found throughout temperate areas of the world and it is the major slug variety found in both Australia and the United Kingdom.
Significant crop damage by molluscs also occurs in northern Europe, the Middle East, North and Central America. South East Asia, Japan and New Zealand. In many cases, the rise to pest status of the slug or snail in question is a consequence of change--either in distribution (as in the case of accidental or deliberate introductions) or in agricultural practice, where new crops or systems of cultivation may enable populations to rise to pest levels. For example, approximately two-thirds of the molluscicides in the United Kingdom are used on winter wheat and winter barley. After harvesting, there is a significant amount of stubble left behind. It is present agricultural practice to drill seeds of the next crop directly into the soil, without removing the stubble of the previous crop by, for example, burning. Slugs, which have buried themselves in the soil, move along into these drill holes and eat the inside out of the new seed, thereby potentially destroying the whole planting. Slugs are therefore a major agricultural pest.
Devising methods of controlling these pests presents a formidable task. Control methods involve cultivation practices, chemical and biological methods. Cultivation procedures that remove or make the habitat of the mollusc less attractive, are usually less expensive. Biological control by introduction of natural predators is a preferred method because, in principle the predator could be snail specific and not harm native snails or non-target organisms. However, very extensive testing is required and, once predators have been introduced, it is very difficult to reverse the process and to remove them. Chemical methods (molluscicides) involve the use of a contact or stomach poison, an irritant or a feeding depressant.
The environment which the mollusc inhabits is generally treated with the molluscicide which is then ingested by the mollusc. Since most snails and slugs thrive in moist conditions, any effective molluscicide should be effective under these conditions. This feature of appropriate water resistance has major implications in broad-acre agriculture, where one treatment is preferred rather than multiple applications throughout the crop season. In this case, it is desirable to have a balance between water resistance and efficacy to prevent the pellets functioning as poisons after the crop has been harvested and livestock has moved into the area to feed. In addition, in areas of very high moisture content there should be effective water-proofing to ensure the poison is maintained in an ingestable form for a sufficient time to permit adequate exposure to the molluscs. Since moisture is essential for slug and snail activity, damage is likely to be more severe on heavy soils due to their greater moisture retention. However, damage is not restricted to heavy soils. Slug and snail activity is encouraged by high levels of organic matter which often provides a moist environment.
Green manure crops and old crop residues used in the compost heap often allow populations to build up quickly. Dense leafy plants, such as brassica and curcubitis, provide a moist humid canopy under which snails and slugs thrive. Temperature also affects the level of slug and snail activity. Indeed, this activity peaks around 15-20.degree. C. and decreases markedly below 5.degree. C. and above 30.degree. C. Furthermore, low temperatures significantly delay the hatching of slug eggs. Most slug and snail species are nocturnal feeders. Hence, watering of gardens in the evening often provides an environment which encourages increased feeding activity.
Molluscicides for use against slugs and snails can be divided into three groups. These are contact-action molluscicides, such as aluminum and copper sulfate crystals, which are applied to the area inhabited by the snail or slug and are taken up passively when the snail or slug moves in this area; irritant powder molluscicides, such as silica grains, which act by being taken up in the snail's or slug's locomotion mucus; and stomach-action molluscicides such as metaldehyde and methiocarb pellets, which are ingested by the mollusc.
Contact-action molluscicides are generally applied in the form of sprays and dusts to crops and the mollusc receives a fatal dose of toxin by moving over the crop. Molluscs present problems of delivery of the toxin because their relatively large size means that a large dose of toxin is necessary. They are also relatively immobile and may remain concealed in comparative safety for long periods. These problems are further complicated by the layer of mucus which invests molluscs. Irritant materials stimulate mucous production and can be sloughed off and left behind in a discarded mucous coat. As the mucus is largely composed of water, the water-solubility of candidate contact poisons is therefore a prerequisite if they are to be able to penetrate the mucous barrier. However, hydropthilic properties in a toxin also increase the rate at which it is diluted by rain and leached into the soil.
Delivery of effective amounts of bait is also a problem. A sufficient amount of poison must be ingested to ensure a lethal dose. In general, most toxic compounds are also repellent and the interaction of toxicity with repellency prevents the ingestion of sufficient poison to kill the mollusc. There are three major effects of molluscs ingesting poison baits. Firstly, there is a possible repellency away from the crop by the bait. Secondly, ingestion of the bait may cause reduced feeding and thirdly, the poison may kill the snail or slug involved.
Until the mid 1960's, the most effective molluscicide was metaldehyde which is a tetramer of acetaldehyde. In Europe, it was known only as a solid fuel, until its molluscicidal properties were discovered accidentally in France by farmers who found dead and dying snails on and around metaldehyde tablets discarded after use in camping stoves. Metaldehyde is toxic at high concentrations and an irritant at lower concentrations, causing mucous secretion and eventual desiccation. A disadvantage is its dependence on thigh temperature and low humidity for its maximum effect and there is a high recovery rate amongst molluscs which are able to reverse the water deficit caused by the excess mucous secretion that metaldehyde stimulates. Under optimal conditions, slugs immobilized and desiccated by metaldehyde will not survive if trapped in the open and exposed to sunlight. Unfortunately, it is under damp conditions and at lower temperatures when metaldehyde is least effective that terrestrial slugs and snails are most active and yet, at higher temperatures, snails are aestivating and not feeding. There is only a very limited period of time during which snails are feeding and the temperature is thigh enough for metaldehyde to be effective.
In the mid 1960s, it was found that carbamate compounds such as methyl carbamate were as toxic to molluscs as metaldehyde. Carbamate compounds cause inhibition of cholinesterases which are the enzymes involved in synaptic nervous transmission in a wide range of animals and their mode of action on insect pests has been extensively studied, particularly in connection with the development of resistance. The methyl carbamate most widely used as a molluscicide is methiocarb (3,5-dimethl-1,4-methylthiophenyl-N-methylcarbamate). The effectiveness of methiocarb is compromised less by low temperatures and high humidity than metaldehyde which is a major advantage, since pest damage often occurs in conditions where metaldehyde is least well suited. However, methiocarb (an active insecticide and acaricide) is more toxic to non-target organisms such as beneficial insects and earthworms than metaldehyde. Although farmers presently tend to use methiocarb, they would prefer not to because of these highly poisonous characteristics and the fact that sheep often graze in areas that require treatment for snails and slugs. For example, in South Australia there are flood-irrigated pastures for sheep and recently, a high incidence of the conical variety of snail, Cochlicella barbara, has been detected. Therefore, any effective molluscicide used under these conditions would have to be effectively water-proofed in addition to not being toxic to the sheep. Methiocarb is effective on Theba pisana but in view of its insecticidal activity and toxicity to earthworms, its use for this snail variety also has severe drawbacks.
There is considerable evidence to indicate that metal salts used as contact poisons are toxic to molluscs (Glen, D. M. and Orsman, I. A., in "Comparison of molluscicides based on metaldehyde, methiocarb and aluminum sulphate," Crop Protection, (1986), 5, 371-375.) In particular, iron and aluminum salts have been investigated in this regard in some detail in the United Kingdom (Henderson et al, "Aluminum(III) and lron(III) complexes exhibiting molluscicidal activity," Australian Patent AU-B-22526/88). These workers concluded that the effectiveness of the molluscicide was dependent on a number of variables but the chelating of the trivalent iron gave very significantly better results than the unchelated salts. In addition, these workers found that the inclusion of the poison in a bait, as a pellet, gave significantly better results than the direct application of molluscicide to the soil or application of the bait as a powder to the soil. Details of the bait formulation were given without discussion of differences that might be expected from other formulations. Such differences are most probably significant in determining the amount of chelate required for effective control. In field conditions, the efficacy and activity of many metal salts is greatly attenuated by both dilution and the metal ions becoming chemically bound in the soil and being unavailable for toxic action. Proposed contact-action metal poisons such as aluminum tris(acetylacetonate) ("Al(acac)").sub.3 are expensive to manufacture and are therefore not economically feasible for use in the home garden or for horticulture or broad-acre application. Various metal salts are marketed as contact molluscicides and are indeed toxic, but it is debatable whether they are effective under field conditions. As contact-action poisons, they are insufficiently persistent and too repellent to be used in baits. For these reasons, molluscicides used against terrestrial (as opposed to aquatic) targets are usually delivered in the form of stomach-action poisons in baits.
One of the other major problems with stomach-action poisons in that they are often consumed by non-target organisms such as domestic animals, birds and children. In normal agricultural and veterinary applications, the preparations are usually very dilute when applied. However, when baits are used this is not the case and there is always a possibility that the bait will be consumed by a non-target organism. Accidental poisoning of non-target organisms is particularly common in the case of snail and slug bait pellets. It is hard to arrive at a reliable figure for poisoning of dogs, cats and native animals, but in Australia about 10,000 poisonings per annum with perhaps as high as 40-50% being fatal is probably a reasonable estimate. A requirement accordingly exists for molluscicides which are effective against snails and slugs, but which substantially minimize the health and environmental risks and cost limitations of the molluscicides currently available on the market.
There are a number of published efficacy trials which indicate that Ferric sodium EDTA (Iron(III) EDTA or ferric EDTA) salt is an effective contact-action molluscicide. Research has been conducted on a number of iron and aluminum compounds as contact poisons against the slug Deroceras reticulatum (Henderson, I. F. and Martin, A. P., in "Control of slugs with contact-action molluscicides," An. Appl. Biol., (1990), 116, 273-278). These workers reported two types of experiments, one in which the slugs were confined to a treated glass surface and one using wet soil in a laboratory test. Unchelated salts were effective poisons when applied to a glass surface, but were rapidly deactivated when applied on wet soil. Chelation of both metals with organic licands retarded the rate of attenuation on wet soil. These workers also reported a field trial in which chelated iron in a broadcast application applied at 40 kg active ingredient per hectare, or in a bait formulation applied at 1.32 kg/ha of active ingredient was effective against Deroceras reticulatum and Arion spp. They concluded that "on the available evidence, the bait formulation was apparently more efficient, an application rate of 1.32 kg active ingredient leaving 586 slugs dead on the surface within three days while with the broadcast formulation applied at 40 kg active ingredient per hectare, only 204 were recorded dead on the surface in the same period." Iron(III) 2,4-pentanedionate appears to be more toxic than Iron(III) EDTA and although it is difficult to quantify the difference, it appears that on wet soil after 10 days, the 2,4-pentanedione is about twice to three times as toxic. Details of the bait formulation were not given but these are most probably significant in determining the amount of chelate required for effective control.